Linux user changed permissions and ownership of shared folder file not created by him

I am using Linux Mint and my workmates are using Windows. We've got a local, shared server (also Linux) for documentation files and a weird thing happened yesterday: a windows user created a file (.odm) and after I changed it, the ownership of the file changed to me and all the other users, including the one who created it, had permission only to read it, although, initially (before I edited it) everyone could read, write and execute. I don't know what information I need to give to make context clearer, but I'd like to understand how that happened. I mean, it seems very weird for a different user to be able to change permissions and ownership of a shared server's file. The server is running samba, and all the clients are using that to access the files.